White light LEDs, which have advantages of low voltage, high light efficiency, low energy consumption, long life and no pollution etc., have been used in fields including semiconductor illumination and liquid crystal flat panel display successfully. Currently, there are two major implementation methods of white light LEDs: one is the combination of LED chips of the three primary colors (red, blue and green) and the other one is a single blue light/ultraviolet chip compound fluorescent material. Between these two methods, the second implementation method, which is simple, easy and relatively cheap, becomes the mainstream solution of white light LEDs. A red fluorescent material, which is an important part in the three primary colors red, green and blue, is indispensable in the white light implementation process. In addition to compensating the lack of red in “blue light LED+YAG:Ce”, the red fluorescent material can be further matched with a blue light LED and a green fluorescent material to generate white light, or matched with green and blue fluorescent materials and a purple light or ultraviolet LED to generate white light.
LED red fluorescent materials which have been reported at present include Eu2+/Eu3+ or Mn4+-activated fluorescent materials, typically (Ca,Sr)S:Eu2+, Y2O3:Eu3+, Bi3+, Y2O2S:Eu3+, Bi3+, Y(V,P)O4:Eu3+ and CaMoO4:Eu3+ etc. Among them, (Ca,Sr)S:Eu2+ has relatively good spectral matching performance with blue light LEDs. However, applications of (Ca,Sr)S:Eu2+ on LEDs are greatly limited due to problems of bad stability and high luminous decay etc. Eu3+ is used as an activator to fluorescent materials including Y2O3:Eu, Bi, Y2O2S:Eu, Bi, Y(V,P)O4:Eu and CaMoO4:Eu. The excitation spectra of these fluorescent materials are some sharp line spectra in long wave ultraviolet of above 370 nm and visible light regions, thereby increasing the difficulty of precise screening and effective control of matched chips. In addition, the excitation efficiency of these several fluorescent materials is extremely low in long wave ultraviolet or visible blue light regions. Although the lighting efficiency of CaMoO4:Eu3+ newly developed in recent years has been improved because of the doping of high concentration of Eu3+, the application of CaMoO4:Eu3+ is also limited greatly by its stringent requirements on chips.
A category of novel nitrogen/nitrogen oxide fluorescent materials have been developed since the late 1990s. Anionic groups of these fluorescent materials contain N3− with high negative charge, and the excitation spectra of the fluorescent materials move towards long wave directions including near ultraviolet and visible lights etc. due to the expansion effect of electronic cloud. In addition, the substrates of the fluorescent materials have compact network structures as well as stable physical and chemical properties, thus leading to enthusiasm for researches on fluorescent materials with nitrogen/nitrogen oxides as substrates.
A category of nitride red fluorescent materials, MxSiyNz:Eu (M is at least one of Ca/Sr/Ba and z=2/3x+4/3y), were disclosed in a patent document EPI 104799A1 in 2001, and there are mainly three typical fluorescent materials MSiN2:Eu, M2Si5N8:Eu and MSi7N10:Eu. This series of red fluorescent materials have relatively bad thermal stability. After the fluorescent materials are heated, their brightness will decrease rapidly.
A category of red fluorescent materials, MaAbDcEdXe, were disclosed in a patent document WO2005/052087 in 2005. In the formula, M is one or two elements of Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, A is one or two elements of Mg, Ca, Sr and Ba, D is one or two elements of tetravalent metal elements Si, Ge, Sn, Ti, Zr and Hf, E is one or two elements of B, Al, Ga, In, Sc, Y, La, Gd and Lu, X is selected from one or two elements of O, N and F. In addition, the red fluorescent materials have a CaAlSiN3 structure and a typical fluorescent material is CaAlSiN3:Eu. The thermal stability of this category of fluorescent materials is obviously higher than that of MxSiyNz:Eu-series (M is at least one of Ca/Sr/Ba and z=2/3x+4/3y) red nitride fluorescent materials, thus arousing widespread attention in the field.
A red fluorescent material having a compositional formula of MmAaBbNn:Zz was disclosed in a patent document CN100340631C in 2005, wherein M is one or more than one element of Be, Mg, Ca, Sr, Ba and Zn, A is one or more than one element of B, Al, Ga, In, Tl, Y and Sc, B is one or more than one element of Si, Ge, Sn and Pb, N is nitrogen, Z is an activator selected from at least one of a rare earth element or a transition element, and (m+z):a:b:n=1:1:1:3. At the same time, the carbon content is limited below 0.08 wt % and the oxygen content is less than 3 wt % in the fluorescent material in the patent.
A red fluorescent material was disclosed in a patent document CN101090953A in 2006. The crystal phase of the fluorescent material is Eu-activated CaAlSiN3. The primary particle size of the fluorescent material is smaller than or equal to 10 μm, and the fluorescent product is not allowed to contain AlN in the patent. The patent simultaneously discloses a raw material and a synthesis method of the fluorescent material. At the same time, the doping concentration of the activator Eu2+ is limited to be 0.01% to 10%.
A (Ca,Sr)AlSiN3:Eu2+ red fluorescent material was also disclosed in a patent document WO2010/074963A1 in 2010. In addition, the contents of impurity oxygen and halogen in the fluorescent material are limited and it is required that the content of impurity oxygen is lower than 2 wt % and the content of halogen (F or/and Cl) is higher than 0 and lower than 2 atomic percent.